The present invention generally relates to fabrication of metallic structures on a substrate and, more particularly, to a process for removing a resist film containing chlorine residue without affecting an underlying organic structure.
The fabrication of passive radio frequency (RF) devices as an on-chip solution for the growing communication industry presents many problems. Problems arise especially when fabricating structures on silicon substrates so as to be compatible with mainstream semiconductor manufacturing techniques. One major problem is RF coupling to the silicon substrate. The silicon substrates used in modern semiconductor production must be isolated from the passive structures as much as possible in order to achieve minimal loss in transmission lines and satisfactory inductor quality factors.
One conventional method of isolation is to place the structures as far from the substrate as possible. This configuration can be achieved by building the structures as the last metal layer and placing them on top of a relatively thick layer of organic polymer, such as polyimide, having a low dielectric constant. The problem with defining metal structures, having small dimensions, on top of an organic material becomes evident when consideration is given to the need to effectively remove the photoresist, which is also an organic material, that defines the structures, without damaging the underlying organic dielectric.
Typically, passive RF transmission lines and inductors are located off chip due to the high losses and poor quality factors of these structures when built on silicon substrates. Placement of these structures over a thick dielectric layer helps to minimize the substrate coupling effects. Although metal lines have been built on top of these organic layers in the past, these have been much larger structures and were able to be fabricated using simple wet etch techniques. As the dimensional requirements have decreased and tolerances have become more critical, the industry is faced with structures small enough that an anisotropic dry or plasma etch must be used.
It has long been known in the art that a chlorine-based plasma etch is very effective for metal reactive ion etching (RIE). This etch has a drawback, however, in that residual chlorine in the photoresist, as well as any underlying organics, will rapidly corrode the metal layer once exposed to atmosphere. This problem is typically resolved by totally stripping the photoresist in a plasma containing oxygen before removing the semiconductor wafers from the vacuum chamber. Although this technique may be effective for removing the photoresist and preventing corrosion, it only works well when the plasma does not affect the underlying layer. When the underlying layer is also an organic material, such as polyimide, it is severely eroded in an isotropic manor. Such erosion is illustrated in FIGS. 2A, 2B, and 2C.
In FIG. 2A, a semiconductor device 200 is shown. An isolation layer of organic material 204 is applied over a substrate 202. A metal layer 206 is then formed over organic layer 204 and a photoresist 208 is patterned over the metal layer 206. In FIG. 2B, the metal layer 206 is etched using a chlorine-based plasma etch. In FIG. 2C, an in situ plasma strip is used to totally remove the photoresist 208 in a single step. This process creates an isotropic erosion 210 under metal layer 206, and may totally undercut metal lines as large as 5 microns. As the dimensions of the metal features get smaller, this undercut problem becomes increasingly critical.
The deficiencies of the conventional photoresist strip methods show that a need still exists for an improved process for removing a resist film containing chlorine residue without affecting an underlying organic structure. The object of the present invention is to overcome the shortcomings of the conventional processes and to meet this need.
Following conventional wisdom, it is believed that the total or near-total strip using a plasma is needed to prevent corrosion. The inventors of the present invention have determined that a much-abbreviated exposure to the plasma may be used to ensure removal of the residual chlorine. This determination leads to a unique combination of plasma strip and solvent etching to remove the photoresist while leaving any underlying organic materials intact.
More specifically, the present invention relates to a process for removing a resist material containing a chlorine residue from an organic substrate. The process comprises the steps of removing the chlorine from the resist material by exposing the resist material to an abbreviated plasma, and removing the remaining resist material by exposing the resist material to a wet solvent.
The present invention further relates to a process for removing a resist material in which about 25% of the resist is striped using a plasma containing oxygen. The present invention still further relates to a process for removing a resist material in which the remaining resist is removed using a wet etch process containing an N-Methyl-2-Pyrrolidone (NMP) etchant, followed by an AZ etchant, which is followed by a further NMP etch. The present invention finally relates to a method of forming a passive RF device over a substrate.
The method comprises the steps of forming an organic layer over the substrate, forming a metal layer over the organic layer, disposing a photoresist over the metal layer, exposing at least a portion of the photoresist to an illumination source to define a pattern of the RF device on the metal layer, removing the unexposed portion of the photoresist from the metal layer to expose at least a portion of the metal layer, etching the portion of the metal layer using an etchant containing chlorine, the etching exposing at least a portion of the organic layer, removing chlorine from the resist material and the exposed portion of the organic layer by an abbreviated exposure to a plasma, and removing the remaining portion of the photoresist material.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.